A thin film transistor including a semiconductor film formed over an insulating surface is an essential semiconductor element for a semiconductor device. Since there is limitation on manufacture of thin film transistors in terms of allowable temperature limit of a substrate, a transistor mainly used for a semiconductor display device is a thin film transistor including amorphous silicon that can be deposited at relatively low temperature, polysilicon that can be obtained by crystallization with use of laser light or a catalytic element, or the like in an active layer.
In recent years, a metal oxide having semiconductor characteristics which is referred to as an oxide semiconductor has attracted attention as a novel semiconductor material which has both high mobility, which is a characteristic of polysilicon, and uniform element characteristics, which is a characteristic of amorphous silicon. The metal oxide is used for various applications; for example, indium oxide is a well-known metal oxide and used as a material of a transparent electrode included in a liquid crystal display device or the like. Examples of such metal oxides having semiconductor characteristics include tungsten oxide, tin oxide, indium oxide, zinc oxide, and the like. A thin film transistor including such a metal oxide having semiconductor characteristics in a channel formation region has been known (Patent Documents 1 and 2).